1. Field of the Invention
The present invention relates to a drier for an oxidative polymerization-drying printing ink, and to a printing ink containing the drier.
2. Description of Related Art
In oxidative polymerization-drying ink such as lithographic offset printing ink, a drier is added to accelerate drying after printing. As the drier, a manganese salt of a fatty acid and a cobalt salt of a fatty acid are commonly used alone or in combination.
Cobalt is classified as a heavy metal and is not innocuous, as is apparent from the data of the evaluation of toxicity in Table 1 shown hereinafter. It can be said that cobalt is a component which imposes an environmental burden among components contained in the lithographic offset printing ink. To obtain an lithographic offset printing ink which is environmentally friendly and contains less toxic component, it would be preferable not to use cobalt. However, when a calcium or iron salt of a fatty acid, which imposes less of an environmental burden and exerts less adverse influence on the environment, is used alone, the effectiveness as a drier is inadequate. Since a substitute therefor cannot be easily found, a drier containing manganese or cobalt is used at present.
An example of data obtained by making a comparison of toxicity between cobalt and manganese is shown in Table 1. As is apparent from the same table, cobalt is more toxic than manganese.
TABLE 1Comparison of toxicity of cobalt and manganese (from the database:REGISTRY OF TOXIC EFFECTS OF CHEMICAL SUBSTANCES)LD50 (rat, oraladministration)Cobalt metal6.17g/kgManganese metal9.0g/kgCobalt naphthenate (popular as drier)3.9g/kgManganese naphthenate (popular as drier)6.0g/kg
In view of increasing concern about environmental problems, the amount of ink using vegetable oil as a raw material has recently increased. A petroleum-based solvent, which has hitherto been used often as a solvent for printing ink, is a typical volatile organic compound (also referred to as “VOC”) and, therefore, trials have been made to replace it with vegetable oil to reduce the amount of the petroleum-based solvent. As used herein, VOC refers to organic compounds having a boiling point within a range from about 50 to 250° C. in accordance with the definition of WHO (World Health Organization) and examples thereof include benzene, toluene and xylene.
For example, printing ink, a VOC component of which is reduced and replaced by soybean oil, is commercially available. Because of poor drying properties compared with conventional ink, this ink is likely to cause a problem referred to as “poor blocking properties” wherein printed surfaces or the printing ink surface and a paper adhere to each other when sheets of printed matter are laid one upon another.